1. Field of Invention
The present invention relates to a fixing device that fixes a fixable medium such as a toner image on a receiving medium such as printing paper, and an image forming device equipped with this fixing device.
2. Description of Related Art
A conventional fixing device includes a heat roller that fixes a toner image transferred to printing paper onto printing paper, and a pressure roller that holds the printing paper between itself and the heat roller. The heat roller includes a hollow cylindrical aluminum roller body, a heater such as a halogen lamp disposed inside the roller body, and a toner parting layer formed over an outer peripheral surface of the roller body and made of a parting agent or release agent that prevents toner from being adhered onto the outer peripheral surface. Thus, when heat is emitted from the heater, that heat is conveyed to the printing paper in contact with the outer surface of the heat roller via the roller body and the toner parting layer, the toner forming the toner image transferred to that printing paper is melted, and the toner image is fixed onto the printing paper.
On the other hand, the pressure roller is formed from a flexible resin such as silicone rubber, and is positioned so as to exert pressure on the heat roller. The pressure roller forms a nip for reliably conveying heat emitted by the heat roller to the printing paper. Thus, while the printing paper is passing through the nip, pressure from the pressure roller is exerted on the heat roller, and as a result, the heat necessary for fixing the toner image onto the printing paper is reliably conveyed from the heat roller to the printing paper.
The fixing device is also equipped with a temperature sensor that monitors the temperature of the outer surface of the heat roller, and a control device that controls the operation of the heater based on the results of monitoring by that temperature sensor, so that the outer surface of the heat roller is at the proper temperature for fixing the toner image onto the printing paper. In this way, the problems known as offset and defective fixing are avoided. Offset refers to a case where the temperature of the heat roller is too high, the toner melts more than is necessary and adheres to the outer surface of the heat roller, the adhering toner circulates around the heat roller, and toner is fixed in areas of the printing paper where toner was not originally transferred. Defective fixing refers to a case where the temperature of the heat roller is too low and toner is not completely fixed onto the printing paper, so that the toner image peels away from the printing paper after printing is completed.
The temperature sensor used in a fixing device may be a contact-type sensor such as a thermistor, or a non-contact type infrared sensor such as a thermopile. The contact-type sensor senses the temperature of the outer surface of the heat roller through direct contact with the outer surface of the heat roller. The non-contact type infrared sensor is positioned at a distance from the heat roller, and receives a bundle of infrared radiation emitted from the outer surface of the heat roller so as to sense the temperature of the outer surface of the heat roller based on the received bundle of infrared radiation.
Which of these is used depends on the design. Generally, however, when the toner that adheres to the surface of the heat roller is so fine that it is not recognizable as offset even if it is fixed onto the printing paper. In this case, if the contact-type sensor is used, toner is scraped off by the sensor, and when the amount scraped off reaches a certain level, it flows out all at once, and the outflowing toner adheres to the printing paper. For this reason, the use of non-contact type infrared sensors as temperature sensors has been proposed in recent years.
A typical thermopile used as a non-contact type infrared sensor has a low heat-resistance temperature, and must be positioned at a certain distance away from the heat roller in order to perform accurate detection of the heat roller temperature. However, when the thermopile is positioned at a distance from the heat roller, objects other than the heat roller fall within the angle of field view of the thermopile. As a result, the thermopile also detects infrared radiation emitted from objects other than the heat roller, in addition to infrared radiation emitted from the heat roller (such infrared radiation which is not the object of detection is hereinafter referred to as a “noise component”). This noise component degrades detection of the heat roller temperature.
Also, the color of the housing of a fixing device is generally black, and the amount of infrared radiation is comparatively large. Therefore, with a fixing device equipped with a black housing, there is a strong tendency for a thermopile to be adversely affected by infrared radiation emitted from other than the heat roller.